Earth-like worlds circling stars in orbital zones suitable for life may be few and far between in the cosmos, according to new research. In the first comprehensive study of extrasolar planetary systems, astronomers have shown that in most of them it would not be possible to keep an Earth-like world in orbit around a star so that it was neither too hot nor too cold for life.

In general, other planetary systems fall into two types: those with Jupiter-like worlds circling close to their parent star, and those with more distant Jupiters in elliptical orbits.

In both systems, maintaining an Earth-like world in a temperate orbit is difficult, although not in all cases impossible.

"This work shows us just how unusual our own Solar System is when compared with the other planetary systems," Dr Kristen Menou of Princeton University, US, told BBC News Online.

Habitable zone

Eighty-five planetary systems were studied, all that were known when the research was carried out.

Dr Menou said: "They fall into two categories: large planets circling very close to their sun - the so-called 'hot Jupiters', and systems with Jupiter-like planets in distant non-circular orbits."

Dr Menou, along with Dr Serge Tabachnik, created computer dynamical models of the known exoplanetary systems to see if it was possible for Earth-like worlds to exist for long periods in the so-called habitable zone.

This work shows us just how unusual our own Solar System is when compared to the other planetary systems

Dr Kristen Menou, Princeton University

This zone is the region around a star in which a planet would be able to sustain liquid water, being neither too close to the star for it all to be vaporised, nor too distant that it all freezes.

In our Solar System, the Earth is in the middle of the habitable zone. Astronomers believe such a position is essential for life to develop and thrive.

But it seems difficult for worlds to stay in the habitable zone in the majority of the extrasolar planetary systems found so far.

"We found that in the systems with the distant Jupiters, these worlds can disrupt the orbit of any Earth-like world in the habitable zone," says Dr Menou.

"Any Earth-like world in the temperate zone would either crash on to its parent star or be slung out into interstellar space," he added.

Over half of the planetary systems studied had distant Jupiters making them unlikely to contain habitable Earth-like worlds.

"We have identified some systems where distant Jupiters would pull Earth-like worlds into elliptical orbits that keep them inside the habitable zone. Such worlds would have dramatic and extreme seasons. We don't know how that would affect the development of life."

Cast asunder

The new analysis of the systems containing hot Jupiters shows that Earth-like worlds could remain orbiting in the temperate zone, seemingly an encouraging finding.

"The good news is that in about a quarter of the systems we studied, there could be habitable planets present."

But even in these systems, Earth-like worlds may have been cast asunder.

Current models of the evolution of planetary systems have hot-Jupiters reaching their tight orbits by migrating inwards from more distant ones.

This means that as they slowly travelled sunwards, they would have scattered any smaller worlds that got in their way, suggesting that there could be no Earth-like worlds in hot Jupiter systems at all.

"The way we are trying to get out of this pessimistic position," says Dr Menou, "is by seeing if Earth-like worlds could form in a planetary system after the inward migration of Jupiter worlds."

The research is to be published in a forthcoming edition of the Astrophysical Journal.

Hmm. The problem with any hypothesis about habitable planets is the technology we use to "discover" the existence of planets orbiting stars is based on the effects those planets have on their stars (wobbly stars, planet transits creating decrease in luminosity, doppler effect of orbiting star).

The methods we use to determine planets have a tendency only to report large (jupiter scale), close-orbiting planets. To my knowledge, even if we were able to go as far away from our own planet as we are from the stars being observed with jupiter like planets, and we looked back at our star, we would probably not detect any planet but jupiter/saturn if that. Our current technology makes it seem like we are alone in the universe but once we get an orbiting inteferometer working we'll be able to identify planets by more than just their effects on their host stars. (we could detect planets by infrared radiation, reflected light, etc., things that we cannot do realistically with 99.999999999999% of the stars out there with current technology) A whole new universe will be ours to see.

I think it is a miracle in itself that planets sustain (nearly) circular orbits- do you have any cluse how little it would take to knock them out of the precise balance of mass/speed that puts it into orbit? (circular OR otherwise)

The fact that there are billions of stars in the universe and we have only detected around 100 of these stars with close, jupiter like planets suggests the opposite of what this article reports. In all likelyhood, the planets we are detecting are the rare ones (big enough to show up after observation with simple telescopes), the more common planets are too small or too far away from their host star to be observable by our current method. Thus, any real studies on the limited data that our technology affords at the moment should be taken as premature, incorrect guesses.

What the article doesn't state though is that planetary systems without Jupiter like outer planets cannot support advanced life either, because the large Jupiter like gas giants are required to keep catastrophic meteors from impacting any life bearing planets in the habital zone.

To our limited knowledge of planetary formation, yes that would make sense, however life-bearing planets could have a possibility of surviving without jovian planets. The thing is they would have to age more (endure a longer period of bombardment) before life really takes off.

Also, I doubt that we can detect jovian planets at the same distance from our sun as jupiter except in the nearest stars.

Yup, and all we need is more sophisticated equipment orbiting somewhere (prefereably a lagrange point) with big enough detectors and an interferometer setup and we might be able to actually see a few blue pixels coming from an earth-like planet. And once we get the light from these planets, we could do a simple spectrum analysis and find out what we have.

This is the central hypothesis of the book "Rare Earth," copyright 2000, Peter D. Ward and Donald Brownlee. Since this contradicts Sagan's hypothesis, the idea was not popular at the time. However, it is gaining acceptance.

In our solar system we have two planets that to an observer light years away might contain life as we know it, Earth and Mars. If our rather ordinary star and solar system has two, I think that with billions of star systems out there, there have to be thousands of planets with temperate stable orbits around stars with the right characteristics for life as we define it. Of course there may be other forms that could exist even in Jovian environments. Maybe the star systems themselves are a life form?

Yes, the Trekkies don't like to even think about it, but most of the stars in the Milky Way can be eliminated from consideration as advanced life bearing systems, because they happen to be in either the core or one of the arms of the galaxy, where the frequency of super novae or interactions with other stars, would extinguish any life. Even if a star resides between a pair of arms it has to be a certain distance from the center of the galaxy or it will eventually drift into one or the other arms. This leave only a small percentage of the stars in the Milky Way that could potentially evolve advanced life forms.

Billions. They have to be rocky planets, otherwise it is unimportant whether they have a main sequence sun or any sun at all. But if you are seeking lifeforms of our level, complex animal/plant lifeforms, pickings are slim to none. Odds are there isn't another earthlike planet in this galaxy, nor in the visible universe out to 14 billion lightyears.

No, not at all. Putting resources into basic research is a necessity, even though no specific results are expected: we learn more about our world and we improve our tools. Who knows how the new knowledge and better tools will be used? The applications can't be known in advance of the advances.

If you want more information on this topic, read anything by Dr. Hugh Ross. A Christian with a background in astrophysics. Basically, the odds of all known required conditions for life to exist coming together at random render the chance of life existing elsewhere in the universe virtually at zero. That doesn't eliminate the possibilty of God creating life elswhere.

This article fails to point out all the other requirements to allow a water-based form of life to develop on a planet.

1) The host star must be absolutely stable for a billion years or so, stable to within a couple of % of nominal, with no excursions.

2) The planet must orbit in the narrow corridor of energy transfer from the star so that the water averages as a liquid.

3) The planet's orbit must be at a sufficient distance so that the solar wind does not blow away the gaseous atmosphere.

4) The planet must have sufficient gravity to keep the atmosphere from drifting off into space.

5) The planet must have water in the first place.

6) The planet must not have a highly eccentric orbit that at anytime would put it outside the corridor (i.e. the more circular the better).

7) The planet must be realtively stable during the same time that the host star is stable.

8) The atmosphere must be balanced in both composition and density to the gravity of the planet so that water can exist in liquid form.

There are a few more, but the idea is that we live on a precarious edge on this planet. We are balanced in every way possible. However, their are no known factors on this planet that could affect that balance. Anything that upsets that balance will come from space, in the form of an asteroid or the Sun having a violent convulsion.

The Sun is expected to swallow up this planet in about 5 billion years as it enters the Red Giant phase of life. It will subsequently convulse, and spew the major portion of its mass into space.

Er, #2 and #3 on your list are essentially counting the same requirement twice (if it's far enough out to prevent the water from boiling off, it's far enough out to prevent the entire atmosphere from being blown off).

Planets weren't put into orbit, they formed there from the disk of debris surrounding the nascent star. The disk is already spinning nearly circular, hence the planetary orbits would be nearly circular.

These are two different requirments that depend on the ratio of energy transfer rate to solar wind bulk density and velocity. There are many combinations that would not work, and each must be satified independently.

Most people are to narrow-minded when it comes to the conditions that life can be sustained. Below are a couple of examples of the harsh conditions it can survive and maybe develope over millions or billions of years.

D. radiodurans can endure 1.5 million rads of radiation, a dose 3000 times higher than would kill organisms from microbes to humans.

Superthermophilic microorganisms inhabit pressurized environments beneath deep-sea hydrothermal vents. These super-organisms not only exist, but thrive at temperatures up to and possibly beyond 150o Centigrade (more than 300o Fahrenheit), setting a new limit at which life can exist.

They're the archaea, an ancient branch of microbial life on Earth discovered by scientists in 1977. Unlike the better known bacteria and eukaryotes (plants and animals), many of the archaea can thrive in extreme environments like volcanic vents and acidic hot springs. They can live without sunlight or organic carbon as food, and instead survive on sulfur, hydrogen, and other materials that normal organisms can't metabolize.

Other bacteria live attached to the siliceous walls of the spring basins, where they are difficult to see, but they can be made visible by a simple trick. If microscope slides are immersed in the boiling water, they serve as surfaces for bacterial colonization.

50-100 organisms survived launch, space vacuum, 3 years of radiation exposure, deep-freeze at an average temperature of only 20 degrees above absolute zero, and no nutrient, water or energy source. (The United States landed 5 Surveyors on the Moon; Surveyor 3 was the only one of the Surveyors visited by any of the six Apollo landings. No other life forms were found in soil samples retrieved by the Apollo missions or by two Soviet unmanned sampling missions, although amino acids - not necessarily of biological origin - were found in soil retrieved by the Apollo astronauts.)

In conclusion, I truely believe life will be found everywhere we look for it.

They are close but the difference would be the passage of time. If the planet went outside the corridor for a short duration each orbit, and the corridor for that planet was relatively wide, a small amount of water could be lost on each excursion, but if the quantity of water was large enough, and it could sustain a billion orbits, there could be an opportunity to have life evolve. So the more circular the better the opportuity.

These are also minimum requirements.

Water has a realtively narrow range of temperature between vapor and solid compared to almost any other substance, So the corridor is going to be pretty narrow for most planets.

I think it is a miracle in itself that planets sustain (nearly) circular orbits- do you have any cluse how little it would take to knock them out of the precise balance of mass/speed that puts it into orbit? (circular OR otherwise)

Considering the mass of even a small planet, LOTS of energy would be required. We couldn't do it with our own Earth if we tried.

What the article doesn't state though is that planetary systems without Jupiter like outer planets cannot support advanced life either, because the large Jupiter like gas giants are required to keep catastrophic meteors from impacting any life bearing planets in the habital zone.

Huh? What space science book did you get this from? I disagree with your statement, though as shoemaker-levy showed us, an outer planet can capture an occasional comet/asteroid.

so far, all the life forms we have found still require water to thrive. Sure, they can aestivate for a while in a dry condition, like a virus, but still need waterborne environment or host to propagate and develop. Water is the key to life.

The tide question is a good one, but I don't think tides would affect the ability for life to develop.

It is interesting that the energy balance of the moon is putting it further from the Earth every year. It is moving away from us about 1 inch per century, and dragging energy from the Earth every month. The result is that the Earth rotational speed is slowing down about (I think) .15 seconds per century. A few hundred million years ago, an Earth day was only 22 hours long.

It is a complex energy transfer, but there are a couple of sites that describe it fairly well.

50-100 organisms survived launch, space vacuum, 3 years of radiation exposure, deep-freeze at an average temperature of only 20 degrees above absolute zero, and no nutrient, water or energy source.

I got a feeling that life survives under conditions and places we would never imagine. We just haven't had the opportunity to discover it yet. We need to leave this little rock we live on and I suspect there will be plenty of surprises.

When I was a kid, I raised some "Sea Monkeys" that arrived in that condition in the mail. But they still needed water to thrive. Yeah, I was about 9 or 10 and thought they would be real monkeys. Worst money I ever spent.

This article is BS, there are unkown amount of stars and to say that Earth is the only one live planet in the known galaxy, is pure and simple BS. God created the universe. He also created many planets.

Yes, the moon is leaving the earth. In a sense it has already left, being in an independent orbit around the sun, but at the same time being resonantly locked with the earth for the time being. The tidal pool hypothesis--a condition that might enhance life-forming conditions--seems like an interesting idea, but probably not a necessary condition.

yes but from that point on, add a little mass or adjustment in speed, and you have a different orbit, right?

I mean the calculation for orbit does not have a 'well' around the path of a stable orbit where a little bit here or there wouldn't matter because you would wobble back into a stable orbit... Or am I missing something?

Yes, the moon is leaving the earth. In a sense it has already left, being in an independent orbit around the sun Um..the moon orbits the earth, it is not in an independent orbit of the sun and it is not "resonantly locked." Examples of resonance are the four largest moons of jupiter, Callisto, Galileo, Europa, and Ganymede. The tidal pool hypothesis is just that. There are more ideas to life creation but until we are able to recreate it all or at least discover the mechanism for genesis. For all we know life could have arrived from a planetesimal striking the earth that was created in the vaccuum of space.

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